The bleaching of cellulosic materials such as wood pulp is carried out in a number of stages. In a typical process, an aqueous slurry of pulp is initially chlorinated with chlorine gas or chlorine water and is then subjected to caustic extraction using sodium hydroxide or the like. Following the chlorination (C) and caustic extraction (E) stages, the pulp is treated with any number of bleaching agents in a variety of sequences selected to achieve a desired brightness, while minimizing viscosity degradation or loss of strength of the pulp fiber. Some of the bleaching agents which have been proposed or used in the past include chlorine dioxide (D), sodium and calcium hypochlorite (H), hydrogen peroxide (P) and hypochlorous acid.
The use of chlorine dioxide is wide-spread, and it is employed in almost all modern pulp bleaching operations. It is also used in many pre-bleaching steps, such as in chlorination or delignification stages to minimize the loss of fiber strength or viscosity. Chlorine dioxide is usually prepared on-site in dilute aqueous solution by the reduction of sodium chlorate in a strong acid solution with a suitable reducing agent.
The process of producing sodium chlorate for the on-site generation of chlorine dioxide has a high energy requirement which makes chlorine dioxide expensive compared to other conventionally used bleaching agents. Because of its expense, every effort is made during use of chlorine dioxide to obtain maximum chemical efficiency. This includes maintaining the pulp slurry at a temperature from 65.degree. to 75.degree. C. for three hours or more in order to maximize the effectiveness of the chemical and achieve the desired brightness. Some bleaching applications require higher temperatures for as long as four hours.
The high operating temperature associated with the use of chlorine dioxide as a bleaching agent is a burdensome expense. Energy costs can vary from $2.00 to $8.00 per million B.T.U.s at the mill site resulting in a cost increase of $1.00 to $3.00 per ton for each 10.degree. C. increase in operating temperature.
It has been theorized that hypochlorous acid is formed intermediately during bleaching of wood pulp with chlorine dioxide. In spite of this possibility, prior art attempts to use hypochlorous acid as a primary bleaching agent that is added to the pulp have been unsuccessful, and no commercial process based on hypochlorous acid stage bleaching is known to exist. U.S. Pat. No. 2,178,696 suggests that the irregular results obtained with hypochlorous acid are caused by "impurities", such as hydrogen chloride, salts of alkali or alkaline earth metals, chlorine dioxide, etc., and proposes the use of "pure" hypochlorous acid to overcome these difficulties.
The prior art has recognized that various additives can be used with the conventional bleaching agents for the purpose of maintaining high pulp viscosities while achieving the desired brightness. Typically, such additives are nitrogen containing compounds, such as sulfamic acid, amino acids, urea, and the like. As previously mentioned, it has also been recognized that chlorine dioxide is effective to protect pulp viscosity during the chlorination state of bleaching.